In the semiconductor industry there is a continuous effort to increase the amount of information that can be stored by an individual memory chip, to decrease the power consumption by the chip and to improve the reliability of the stored information. The effort to increase the memory storage necessarily dictates a corresponding effort to reduce the size of the memory cell. Reducing the size of the memory cell and the power consumption are not always consistent with the goal of improving reliability of the stored information.
The standard static random access memory (SRAM) cell is a six transistor cell. The only way to reduce the size of this cell is to shrink or reduce the design rules; that is, to reduce the minimum allowable size of elements or spacing between elements. Reducing the design rules increases manufacturing difficulty and reduces reliability. One alternative proposal for reducing memory size has been to replace the six transistor cell with a cell comprising one transistor and two negative differential resistance (NDR) diodes. Unfortunately, to date the available NDR diodes provide a peak to valley current ratio of only ten or less which is insufficient for large scale SRAM applications. In addition, the available NDR diodes have required thin alternating layers of compound semiconductor materials deposited by molecular beam epitaxy. Such diodes cannot be easily and inexpensively integrated into large scale memories.
Accordingly, it is desirable to provide an NDR diode that can be easily manufactured and integrated into a large scale integrated circuit. In addition, it is desirable to provide a memory cell that includes NDR diodes that can be readily manufactured, has a small “footprint”, reduces power consumption, and is stable and reliable. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.